1. Field of the Invention
The present invention relates to a solid-state image pickup device and an image pickup apparatus that convert an analog image signal into digital image data and output the digital image data.
2. Description of the Related Art
As a solid-state image pickup device for use in an image pickup apparatus, there has been provided a complementary metal oxide semiconductor (CMOS) image sensor. A CMOS image sensor includes, for example, a photodiode that performs photoelectric conversion for each of a plurality of pixels constituting a pixel array unit, a read-out transistor that reads out signal charges generated by the photodiode, an amplification transistor that converts the read-out signal charges into a pixel signal, a reset transistor that resets signal charges, and a selection transistor that selects a pixel to be read out. A known CMOS image sensor of this type has a peripheral circuit capable of performing various processing on an image signal output from a pixel array unit and converts, in the solid-state image pickup device, the processed image signal into digital image data to output the digital image data.
There have also been provided various image pickup apparatuses, such as camera apparatuses, in which even if a solid-state image pickup device does not have a digitizing function, an analog image signal output from the solid-state image pickup device is digitized by an external circuit, subjected to various digital processing, and output. In this case, various analog-output-type CMOS image sensors and charge-coupled device (CCD) image sensors can be used as the solid-state image pickup device. Also, there has been provided an image pickup apparatus that includes a sensor module serving as an intermediate module between an image sensor and a camera apparatus. The sensor module is provided as a package including a CMOS image sensor or CCD image sensor and an external circuit.
For solid-state image pickup devices of the types described above, it is necessary to be capable of removing dark current. To deal with variations in dark current among devices and fluctuations in dark current that occur during an image pickup operation, there are provided various methods in which, for example, a black reference signal detected from light-shielding pixels (or an optical black (OB) area) provided in part of a pixel array unit is properly corrected and clamped to a target value.
For example, there is proposed a configuration in which an analog black reference signal included in an input signal is detected and compared with a target value, an error signal is calculated from the result of the comparison, feedback is applied to the error signal, the input signal is corrected with the error signal, and an appropriate correction value is calculated by digital computation (see, e.g., Japanese Unexamined Patent Application Publications Nos. 10-276976 and 2005-80263).
There is also proposed a configuration in which processing similar to that described above is achieved without performing feedback of an analog signal, and is achieved only by performing a clamp correction of a digital signal. Thus, a circuit is simplified (see, e.g., Japanese Unexamined Patent Application Publications Nos. 10-174002 and 2004-129000).